[Technical Field]
CROSS REFERENCE TO RELATED APPLICATION(S)
[0002] The present invention relates to a multi-layered electrode for a rechargeable battery
including a binder having high crystallinity.
[Background Art]
[0003] As technical development and demand for mobile devices have increased, demand for
rechargeable batteries as an energy source has rapidly increased. Among such rechargeable
batteries, lithium rechargeable batteries having high energy density and operating
potential, having a long cycle life, and having a low self-discharge rate have been
commercialized and widely used.
[0004] Also, recently, as the interest on environment issues has grown, research into electric
vehicles (EV), hybrid electric vehicles (HEV), and the like, which may replace vehicles
based on fossil fuel, such as gasoline vehicles, diesel vehicles, which is one of
the main causes of air pollution, has actively conducted. As a power source of the
electric vehicles (EV), hybrid electric vehicles (HEV), and the like, lithium rechargeable
batteries having high energy density, high discharge voltage, and output stability
are mainly studied and used.
[0005] However, in such a development orientation, battery stability has been reduced, and
thus, there have been attempts to solve the problem.
[0006] For example, if a battery pack is penetrated due to an external impact or external
deformation, electrochemical energy inside the battery is converted into thermal energy,
causing rapid heat generation, and ensuing heat causes a positive electrode or negative
electrode material to make a chemical reaction, which causes a rapid exothermic reaction
to cause the battery to be ignited or exploded, causing a stability problem.
[0007] In particular, it is known that an explosion due to needle-shaped penetration, or
the like, occurs due to local IR-heat due to a short-circuit current due to contact
between a nail and a current collector or between an electrode material and the current
collector inside the battery.
[0008] That is, the local short-circuit causes an excessive current to flow, and the current
causes heat generation. A magnitude of the short-circuit current due to the local
short-circuit is in inverse proportion to resistance, and thus, the short-circuit
current mostly flow to a side where resistance is low, and here, the current flows
through a metal foil used as a current collector, and calculation of heat generation
at this time shows that strong heat generation locally occurs around a portion penetrated
by the nail.
[0009] Also, when heat generation occurs inside the battery, a separator contracts to cause
a short-circuit between the positive electrode and the negative electrode, and repeated
heat generation and contraction of the separator increase short-circuit sections to
cause thermal runaway or cause the positive electrode, the negative electrode, and
an electrolyte forming the inside of the battery to react each other or to be burnt.
Here, the reaction is a huge exothermic reaction, so the battery may be ignited or
exploded. Riskiness is a more important issue especially as lithium rechargeable batteries
have higher capacity and energy density is increased.
[0010] In addition, in the case of a battery module or a battery pack designed to provide
high output large capacity using multiple batteries as unit cells, the aforementioned
stability issue may be more serious.
[0011] In order to solve the problem and enhance stability, in the related art, a material
having high heat conductivity, a fire-proof material, or the like, is adhered to a
pouch so that the other material may be penetrated first before the needle-shaped
penetration, thus preventing overheating or ignition. This method, however, includes
an additional process and incurs additional cost when manufacturing a rechargeable
battery, increases a volume of the rechargeable battery, and reduces capacity per
unit volume.
[0012] Therefore, the necessity for a rechargeable battery which may have enhanced stability
and manufactured without an additional process or material is high.
[0013] Document
JP 2010 282873 A discloses a laminated electrode structure, wherein the crystallinity degree of the
binder (PVdF) in the layer closer to the current collector is higher than the crystallinity
degree of the binder closer to the electrolyte, and a method of manufacturing the
same. Document
KR 2015 0028663 A discloses an electrode, the method of manufacturing such electrode and a lithium
rechargeable battery including it, wherein the electrode has two electrode active
material layers, each including an electrode active material, a binder (such as PVdF)
and a conductive material. Documents
KR 2014 0132792 A and
JP 2015 076248 A disclose an electrode containing a current collector coated with an active material
and a semi-crystalline PVdF binder obtained by a heating process.
[DISCLOSURE]
[Technical Problem]
[0014] The present invention has been made in an effort to solve the aforementioned problems
of the related art and technical problems requested from the past.
[0015] The inventors of the present application repeatedly conducted in-depth study and
various experimentations to discover that the use of a binder having a high crystallinity
in a partial electrode composite layer forming a multilayer electrode reduces an elongation
percentage of the electrode to achieve a desired effect, thus completing the present
invention.
[Technical Solution]
[0016] According to the invention it is provided an electrode for a rechargeable battery
in which a current collector is coated with an electrode mixture including an electrode
active material and a binder, including:
a first electrode composite layer including PVdF as a first binder and the electrode
active material and applied on a current collector; and
a second electrode composite layer including a second binder and an electrode active
material and applied on the first electrode composite layer,
wherein crystallinity of the first binder is 58 or greater.
[0017] Here, the second binder may be, but not limited to, the same PVdF as the first binder,
and here, the second binder has crystallinity of less than 58.
[0018] The crystallinity, which represents a weight ratio of a crystal part in the entirety
of a polymer solid including the crystal part and a non-crystal part, is changed depending
on a type and a structure of a polymer and varied depending on a crystallization temperature,
a cooling rate, an external force, and the like.
[0019] A method for measuring the crystallinity may include a density method which obtains
crystallinity from two densities of the crystal part and the non-crystal part on the
assumption of additive properties, a method based on measurement of heat of fusion,
an X-ray method for obtaining crystallinity by dividing a strength distribution of
an X-ray diffraction into a diffraction based on the non-crystal part and a diffraction
based on a crystal part, an infrared ray method for obtaining crystallinity from a
strength of a crystallinity band width of an infrared absorption spectrum, and the
like. Crystallinity according to the present invention refers to a result obtained
by measuring crystallinity by the X-ray method, in particular, an NMR measurement
method.
[0020] In a general electrode according to the invention, crystallinity of the PVdF measured
by the above method is less than 58. This is because, the electrode is easily broken
as the crystallinity of the PVdF is higher, and thus, if the crystallinity of the
PVdF is too high, resistance is increased to cause a problem of an output, or the
like.
[0021] Meanwhile, the inventors of the present application repeatedly conducted in-depth
study to discover that safety of needle-shaped penetration of the electrode may be
increased using such characteristics of the PVdF.
[0022] In detail, if the electrode layer includes only the PVdF having crystallinity of
58 or greater, flexibility of the electrode may be so low that resistance is increased
and output characteristics are significantly reduced as mentioned above. Therefore,
the inventors of the present application manufactured an electrode including two layers,
in which a binder formed of PVdF and having a crystallinity of 58 or higher is used
as a first binder in a first electrode composite layer coated on a current collector
and a binder having a crystallinity of less than 58 as a second binder in a second
electrode composite layer coated on the first electrode composite layer, thus reducing
an elongation percentage of the electrode layer, without significantly degrading output
characteristics, thus enhancing safety of needle-shaped penetration.
[0023] That is, since the electrode for a rechargeable battery having such a structure has
a low elongation percentage, a short-circuit area of the current collector and the
electrode material at the time of needle-shaped penetration may be reduced, obtaining
the above-mentioned effect.
[0024] Also, according to the present invention, the crystallinity of the PVdF may be adjusted
very simply and easily by regulating a drying temperature of the electrode.
[0025] In this connection, in order to enhance safety of needle-shaped penetration, in the
related art, methods such as forming a separate ceramic powder coating layer, coating
a material having a high elongation percentage on a pouch, or the like, have been
proposed, but these methods inevitably use or include an additional material or additional
process. In contrast, according to the present invention, the effect may be obtained
using the electrode material used in the existing case as is and differentiating only
a drying temperature, and thus, material cost and process efficiency are excellent.
[0026] Meanwhile, in order to prevent a degradation of the output characteristics of the
electrode, while exhibiting the above-mentioned effect, a thickness of the first electrode
composite layer is smaller than that of the second electrode composite layer, and
specifically, the thickness of the first electrode composite layer may be 5 to 45%,
more specifically, 5 to 30% with respect to the thickness of the second electrode
composite layer.
[0027] If the thickness of the first electrode composite layer is too small to be outside
the range, it is not sufficient to reduce the elongation percentage of the electrode,
resulting in failure to ensure desired safety of the needle-shaped penetration, and
if the thickness is too large, the overall electrode may be easily broken and resistance
is increased to degrade the output characteristics, which is, thus, not desirable.
[0028] The electrode active material included in the first electrode composite layer and
the second electrode composite layer is not limited to the known active materials.
[0029] Specifically, when the electrode for a rechargeable battery is a positive electrode,
the electrode active material may include, as a positive electrode active material,
for example, a layered compound such as lithium cobalt oxide (LiCoO
2), lithium nickel oxide (LiNiO
2), and the like, or a compound substituted to a transition metal of 1 or greater;
a lithium manganese oxide such as a chemical formula Li
1+xMn
2-xO
4 (here, x is 0 to 0.33), LiMnO
3, LiMn
2O
3, LiMnO
2, and the like; lithium copper oxide (Li
2CuO
2); vanadium oxide such as LiV
3O
8, LiFe
3O
4, V
2O
5, CU
2V
2O
7; Ni site type lithium nickel oxide represented by chemical formula LiNi
1-xM
xO
2 (here, M = Co, Mn, Al, Cu, Fe, Mg, B, or Ga and x = 0.01 to 0.3); a lithium manganese
composite oxide represented by chemical formula LiMn
2-xM
xO
2 (here, M = Co, Ni, Fe, Cr, Zn, or Ta and x = 0.01 to 0.1) or Li
2Mn
3MO
8 (here, M = Fe, Co, Ni, Cu, or Zn); lithium manganese composite oxide having a spinel
structure represented by LiNi
xMn
2-xO
4; LiMn
2O
4 in which a portion of Li of chemical formula is substituted with alkaline earth metal
ion; disulfide compound; Fe
2(MoO
4)
3, and the like, but is not limited thereto.
[0030] Meanwhile, when the electrode for a rechargeable battery is a negative electrode,
the electrode active material may include, as a negative electrode active material,
for example, at least one carbon-based material selected from the group consisting
of crystalline artificial graphite, crystalline natural graphite, amorphous hard carbon,
low crystalline soft carbon, carbon black, acetylene black, Ketjenblack, Super P,
graphene, and fibrous carbon, Si-based material, Li
xFe
2O
3(0≤x≤1), Li
xWO
2(0≤x≤1), Sn
xMe
1-xMe'
yO
z(Me: Mn, Fe, Pb, Ge; Me': Al, B, P, Si, group 1, group 2, group 3 elements of the
periodic table, halogen; metal composite oxide such as 0≤x≤1; 1≤y≤3; 1≤z≤8); lithium
metal; lithium alloy; silicon-based alloy; tin-based alloy; metal oxide such as SnO,
SnO
2, PbO, PbO
2, Pb
2O
3, Pb
3O
4, Sb
2O
3, Sb
2O
4, Sb
2O
5, GeO, GeO
2, Bi
2O
3, Bi
2O
4, and Bi
2O
5; conductive polymer such as polyacetylene; Li-Co-Ni-based material; titanium oxide;
lithium titanium oxide, and the like, but is not limited thereto.
[0031] Also, here, the kind of the electrode active material may be different in the first
electrode composite layer and the second electrode composite layer, but may be the
same specifically in terms of manufacturing process.
[0032] In general, safety of needle-shaped penetration may be increased if a short-circuit
area is reduced by lowering an elongation percentage of only any one of the positive
electrode and the negative electrode. However, the negative electrode generally uses
a Cu foil as a current collector, so it has an elongation percentage higher than that
of the positive electrode which uses an Al foil as a current collector. Thus, although
the elongation percentage is reduced by the method according to the present invention,
there is a limitation in reducing the short-circuit area, and thus, reducing the elongation
percentage of the positive electrode is more effective for reducing a short-circuit
current.
[0033] Therefore, the electrode for a rechargeable battery may be, specifically, a positive
electrode.
[0034] Meanwhile, the content of the first binder and the content of the second binder respectively
included in the first electrode composite layer and the second electrode composite
layer may be 1 to 15 wt% with respect to a total weight of each of the electrode composite
layers.
[0035] If the content of each of the binders is too low to be outside the range, adhesion
between the current collector and the active material or between the active materials
is lowered so the desired effect of the present invention cannot be obtained, and
if the content of each of the binders is too high, resistance in the electrode may
be increased to degrade the characteristics of the battery, and since the content
of the active material and other electrode materials is relatively low, capacity and
conductivity of the electrode are lowered, which are, thus, not desirable.
[0036] In addition, each of the electrode composite layers may include various copolymers
of one or more monomers selected from the group consisting of polyvinylidene fluoride,
polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated
cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene,
ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butadiene rubber,
fluorine rubber, or styrene monomer (SM), butadiene (BD), and butyl acrylate (BA),
as additional binding agents, in addition to the first binder and the second binder.
[0037] The first electrode composite layer and the second electrode composite layer may
include a conductive material having electronic conductivity to enhance conductivity.
[0038] The conductive material is not particularly limited as long as it has conductivity
without causing a chemical change in the corresponding battery. For example, a conductive
material such as graphite such as natural graphite, artificial graphite, and the like;
carbon black such as acetylene black, ketjen black, channel black, furnace black,
lamp black, summer black, and the like; conductive fiber such as carbon fiber, metal
fiber, and the like; metal powder such as carbon fluoride, aluminum, nickel powder,
and the like; conductive whiskey such as zinc oxide, potassium titanate, and the like;
conductive metal oxide such as titanium oxide, and the like; conductive materials
such as polyphenylene derivatives, and the like, may be used. Specific examples of
the conductive material on the market include Chevron Chemical Company or denka black
(Denka Singapore Private Limited) of an acetylene black line, Gulf Oil Company product,
etc., Ketjenblack, EC line (Armak Company) product), Vulcan XC-72 (Cabot Company)
product) and Super P (Product of Timcal company), and the like.
[0039] Here, the content of the conductive material may be 20 parts by weight to 100 parts
by weight against 100 parts by weight of the first binder and the second binder.
[0040] If the content of the conductive material is less than 20 parts by weight so as to
be outside the range, a desired degree of conductivity may not be obtained, and if
the content of the conductive material exceeds 100 parts by weight, the content of
the active material is relatively reduced to reduce capacity, which is, thus, not
desirable.
[0041] In some cases, a filler, as a component suppressing expansion of the electrode, may
be selectively added. The filler is not particularly limited as long as it is a fibrous
material without causing a chemical change in the corresponding battery, and may be,
for example, an olefin polymer such as polyethylene, polypropylene, and the like;
and a fibrous material such as glass fiber and carbon fiber.
[0042] Also, other components such as a viscosity controlling agent, an adhesion promoter,
and the like, may be further included selectively or as a combination of two or more
thereof.
[0043] The viscosity controlling agent, as a component for controlling viscosity of an electrode
mixture to facilitate a mixing process of the electrode mixture and a coating process
thereof on the current collector, may be added in the amount of 30 wt% with respect
to the total weight of the electrode mixture. The viscosity controlling agent may
be, for example, carboxymethylcellulose, polyvinylidene fluoride, and the like, but
is not limited thereto. In some cases, the aforementioned solvent may also serve as
the viscosity controlling agent.
[0044] The adhesion promoter, as an auxiliary component added to enhance adhesion of the
active material to the current collector, may be added in the amount of 10 wt% or
less against the binder. The adhesion promoter may include, for example, oxalic acid,
adipic acid, formic acid, an acrylic acid derivative, an itaconic acid derivative,
and the like.
[0045] Meanwhile, the current collector of the electrode for a rechargeable battery according
to the present invention may have a thickness of 3 to 500
µm. The current collector is not particularly limited as long as it has conductivity
without causing a chemical change in the corresponding electrode. For example, the
current collector may be formed of copper, stainless steel, aluminum, nickel, titanium,
and sintered carbon, or copper, aluminum, stainless steel surface-treated with carbon,
nickel, titanium, silver, and the like, or an aluminum-cadmium alloy, and the like.
The current collector may have fine protrusions and depressions formed on a surface
thereof to enhance adhesion of the electrode active material, and may have various
forms such as a film, a sheet, a foil, a net, a porous body, foam, non-woven fabric,
and the like.
[0046] The present invention further provides a method for manufacturing an electrode for
a rechargeable battery according to the present invention.
[0047] First, the electrode for a rechargeable battery including a first electrode composite
layer and a second electrode composite layer according to the present invention may
be manufactured, for example, by a method including:
- (i) applying a slurry including PVdF as a first binder and an electrode active material
to a current collector, subsequently first drying the slurry at 120 to 140°C under
an air atmosphere for 2 minutes to 5 minutes, and secondly drying the slurry at 150
to 190°C in a vacuum state for 12 hours to 30 hours to form a first electrode composite
layer; and
- (ii) applying a slurry including a second binder and an electrode active material
to the first electrode composite layer and subsequently drying the slurry at 120 to
140°C under an air atmosphere for 2 minutes to 5 minutes and rolling the dried slurry
to form a second electrode composite layer.
[0048] As mentioned above, crystallinity of PVdF according to the present invention may
be adjusted by regulating a drying temperature of the electrode.
[0049] In detail, the crystallinity of the PVdF is increased as the vacuum drying temperature
is increased. Thus, in addition to first drying the first electrode composite layer
to volatilize NMP, the second vacuum drying temperature is a temperature higher than
130°C, which is a general electrode drying temperature, that is, 150 °C to 190°C,
specifically, 160°C to 190°C.
[0050] If the second drying temperature is too low to be outside the range, the desirable
crystallinity of the PVdF cannot be obtained, and if the second drying temperature
is too high, the other electrode materials may be changed in characteristics or broken,
which is, thus, not desirable.
[0051] The first drying of the slurry for forming the first electrode composite layer, as
a process for volatilizing NMP, is performed for about 2 minutes to 5 minutes, and
the second drying, which aims at increasing crystallinity of the PVdF, is performed
for about 12 hours to 30 hours.
[0052] Also, the drying temperature of the second electrode composite layer is a general
electrode drying temperature similar to that of the related art, i.e., 120°C to 140°C,
and specifically, 130°C. In this case, since crystallinity of the second binder is
maintained to be less than 58, the entire electrode may not have characteristics of
being easily broken, and since resistance is not increased, a degradation of output
characteristics may be prevented. Here, drying of the second electrode composite layer,
also as a process for volatilizing NMP, is performed for about 2 minutes to 5 minutes.
[0053] As the coating method, drying, rolling, and the like, coating, drying, rolling, and
the like, of the electrode manufacturing method known in the art may be applied without
any particular limitation.
[0054] The electrode manufacturing method according to the present invention may be changed
in a partial process as necessary, and these should be interpreted to be included
in coverage of the present invention. For example, rolling may be performed during
the process of forming each electrode composite layer.
[0055] The electrode for a rechargeable battery according to the present invention may be
used in a lithium rechargeable battery.
[0056] The lithium rechargeable battery may have a structure in which an electrode assembly
including electrodes, i.e., a positive electrode and a negative electrode and a separator
interposed therebetween is filled with lithium salt-containing non-aqueous electrolyte.
[0057] The separator is interposed between the positive electrode and the negative electrode
and may be an insulating thin film having high ion permeability and mechanical strength.
A diameter of a pore of the separator is generally 0.01 to 10
µm and a thickness thereof is generally 5 to 300
µm. As the separator, a sheet or non-woven fabric formed of an olefin polymer such
as polypropylene having chemical resistance and hydrophobic properties, glass fiber,
polyethylene, or the like, is used
[0058] In some cases, the separator may be coated with a gel polymer electrolyte to enhance
stability of the battery. Typical gel polymers include polyethyleneoxide, polyvinylidenefluoride,
polyacrylonitrile, and the like. When a solid electrolyte such as a polymer, or the
like, is used as the electrolyte, the solid electrolyte may also serve as the separator.
[0059] The lithium salt-containing non-aqueous electrolyte may include a non-aqueous electrolyte
and lithium salt, and the non-aqueous electrolyte includes a non-aqueous organic solve,
an organic solid electrolyte, an inorganic solid electrolyte, and the like, but is
not limited thereto.
[0060] Examples of the non-aqueous organic solvent may include aprotic organic solvent such
as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate,
dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, gamma-butylolactone,
1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran,
dimethylsulfoxide, 1,3-diosolane, 4-methyl-1,3-dioxen, diethyl ether, formamide, dimethyl
formamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate,
triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane,
1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives,
ether, methyl propionate, ethyl propionate, and the like.
[0061] The organic solid electrolyte may include, for example, a polymeric material including
polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives,
a phosphate ester polymer, poly agitation lysine, polyester sulfide, a polyvinyl alcohol,
a polyvinylidene fluoride, ionic dissociation group, and the like.
[0062] The inorganic solid electrolyte may include, for example, nitride of Li such as Li
3N, LiI, Li
5NI
2, Li
3N-LiI-LiOH, LiSiO
4, LiSiO
4-LiI-LiOH, Li
2SiS
3, Li
4SiO
4, Li
4SiO
4-LiI-LiOH, Li
3PO
4-Li
2S-SiS
2, and the like, halide, sulfate, and the like.
[0063] The lithium may be a material easily dissolved in the non-aqueous electrolyte and
include, for example, LiCI, LiBr, Lil, LiClO
4, LiBF
4, LiB
10Cl
10, LiPF
6, LiCF
3SO
3, LiCF
3CO
2, LiAsF
6, LiSbF
6, LiAlCl
4, CH
3SO
3Li, CF
3SO
3Li, LiSCN, LiC(CF
3SO
2)
3, (CF
3SO
2)
2NLi, lithium chloroborane , lower aliphatic carboxylic acid lithium, lithium 4-phenylborate,
imide, and the like.
[0064] Also, in order to improve charge/discharge characteristics, flame retardancy, and
the like, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine,
n-glyme, hexafluorophosphoric triamide, nitrobenzene derivatives, sulfur, quinone
imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol
dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, trichloro-aluminum, and the
like, may be added to the lithium salt-containing non-aqueous electrolyte. In some
cases, in order to impart nonflammability, a halogen-containing solvent such as carbon
tetrachloride, ethylene trifluoride, and the like, may be further included, and in
order to enhance high-temperature storage characteristics, a carbon dioxide gas may
be further included.
[0065] In a specific example, a lithium salt-containing non-aqueous electrolyte may be manufactured
by adding lithium salt such as LiPF
6, LiClO
4, LiBF
4, LiN(SO
2CF
3)
2, and the like, to a mixture solvent of cyclic carbonate of EC or PC, as a high dielectric
solvent and linear carbonate of DEC, DMC, or EMC as a low viscosity solvent.
[Mode for Invention]
[0066] Hereinafter, the present invention will be described in detail through embodiments,
but the embodiments below are provided to exemplify the present invention and scope
of the present invention is not limited thereto.
<Comparative Example 1>
[0067] LiNi
0.3Co
0.3Mn
0.3O
2 was used as a positive electrode active material. 96 wt% of LiNi
0.3Co
0.3Mn
0.3O
2 and 2.0 wt% of Super-P (conductive material), and 2.0 wt% of PVdF (first binder)
were added to N-methyl-2-pyrrolidone (NMP) which is a solvent to prepare a first positive
electrode active material slurry.
[0068] The first positive electrode active material slurry was applied on an aluminum foil
to have a thickness of 50
µm, and dried at a rate of 0.2 m/min. (rate of drying for about 5 minutes) in a dryer
under an air atmosphere of 130°C for NMP drying to form a first positive electrode
composite layer, and a second positive electrode active material slurry obtained by
adding LiNi
0.3CO
0.3Mn
0.3O
2 as a positive active material, Super-P as a conductive material, and PVdF as a binder
in a weight ratio of 96:2:2 to NMP was applied to have a thickness of 100
µm on the first positive electrode composite layer and dried at a rate of 0.2 m/min.
(rate of drying for about 5 minutes) in a dryer having a temperature of 130°C under
an air atmosphere to form a second positive electrode composite layer, and thereafter,
the second positive electrode composite layer was rolled to form a positive electrode.
<Comparative Example 2>
[0069] A positive electrode was manufactured in the same manner as that of Comparative Example
1, except that the first positive electrode active material slurry prepared in Comparative
Example 1 was applied to have a thickness of 50
µm on an aluminum foil and NMP was dried at a rate of 0.2 m/min. in a dryer under an
air atmosphere having a temperature of 130°C and dried again for 24 hours at 130°C
in a vacuum state to form a first positive electrode composite layer.
<Inventive Example 1>
[0070] A positive electrode was manufactured in the same manner as that of Comparative Example
1, except that the first positive electrode active material slurry prepared in Comparative
Example 1 was applied to have a thickness of 50
µm on an aluminum foil and NMP was dried at a rate of 0.2 m/min. in a dryer under an
air atmosphere having a temperature of 130°C and dried again for 24 hours at 160°C
in a vacuum state to form a first positive electrode composite layer.
<Inventive Example 2>
[0071] A positive electrode was manufactured in the same manner as that of Comparative Example
1, except that the first positive electrode active material slurry prepared in Comparative
Example 1 was applied to have a thickness of 50
µm on an aluminum foil and NMP was dried at a rate of 0.2 m/min. in a dryer under an
air atmosphere having a temperature of 130°C and dried again for 24 hours at 190°C
in a vacuum state to form a first positive electrode composite layer.
<Experimental Example 1>
[0072] Crystallinity of the PVdF of each the first positive electrode composite layers and
elongation percentage and flexibility of the electrodes in the positive electrodes
manufactured in Comparative Examples 1 and 2 and Inventive Examples 1 and 2 were measured
and illustrated in Table 1 below. To this end, electrodes in which only the positive
electrode composite layer was formed in Comparative Examples 1 and 2 and Inventive
Examples 1 and 2 were separately prepared.
[0073] Here, the crystallinity, elongation percentage, and flexibility were measured in
the following manner.
[0074] *Crystallinity: Electrodes in which only the first positive electrode composite layer
was formed were vacuum-dried at 45°C and a portion of each electrode layer was scraped
with a razor blade, and NMR of powder was subsequently measured. An analysis method
is as follows. After measurement, areas of peaks of crystalline and non-crystalline
at main peaks of the PVdFs are obtained and a percentage (%) of the area of the crystalline
in the sum of the areas is calculated to obtain crystallinity.
[0075] A used device was Agilent 600MHz NMR/1.6mm MAS probe.
[0076] *Elongation percentage: Electrodes in which only the first positive electrode composite
layer is formed are manufactured in the form of a dogbone, and the dogbone is pulled
out at a rate of 5 mm/min. using UTM equipment (INSTRON- Electromechanical 3300),
and an elongated length before the sample is broken is measured.
[0077] *Flexibility: A bar is manufactured for each pi, and the electrodes in which only
the first positive electrode composite layer is formed is cut to have a width of 10
cm and a length of 30 cm. The cut electrode was bent in half, the bar was brought
into contact therewith and both ends of the electrodes are lifted at a rate of 10
mm/min. Here, the both ends of the electrode are lifted until a force measured in
the UTM reaches 5N. The electrode was measured for each pi to observe whether cracks
are formed through an optical microscope, and if there is no crack, testing is performed
with a smaller pi.
(Table 1)
Additional dying temperature (°C) |
Crystallinity |
Elongation percentage (%) |
flexibility(
) |
- (Comparative Example 1) |
48.2 |
1.77 |
5 |
130°C (Comparative Example 2) |
56.1 |
1.52 |
5 |
160°C (Example 1) |
59.7 |
1.33 |
6 |
190°C (Example 2) |
61.1 |
1.25 |
6 |
<Comparative Example 3>
[0078] The first positive electrode active material slurry of Comparative Example 1 was
applied to have a thickness 150
µm on an aluminum foil and dried at a rate of 0.2 m/min. (rate of drying for about
5 minutes) in a dryer having a temperature of 130°C under an air atmosphere to form
a positive electrode composite layer, and the positive electrode composite layer was
then rolled to manufacture a positive electrode.
<Experimental Example 2>
Manufacturing of Negative Electrode
[0079] Artificial graphite was used as a negative electrode active material. A negative
electrode active material slurry prepared by adding 96.3 wt% of artificial graphite,
1.0 wt% of Super-P (conductive material), and 2.7 wt% of PVdF (bonding agent) to NMP
as a solvent was applied to have a thickness of 70
µm on a copper foil and dried at a rate of 0.2 m/min. (rate of drying for about 5 minutes)
in a dryer having a temperature of 130°C under an air atmosphere and rolled to manufacture
a negative electrode.
Manufacturing of Rechargeable Battery
[0080] Rechargeable batteries were manufactured using the positive electrodes and negative
electrodes manufactured in Inventive Examples 1 and 2 and Comparative Examples 1 to
3, a poly ethylene film (Celgard, thickness: 20
µm) as a separator, and a liquid electrolyte in which 1M of LiPF
6 was dissolved in a solvent obtained by mixing ethylene carbonate, dimethylene carbonate,
and diethyl carbonate in a ratio of 1:2:1.
Experiment of Safety of Nail Penetration
[0081] Five rechargeable batteries manufactured using the positive electrodes of Inventive
Examples 1 and 2 and Comparative Examples 1 to 3 were prepared to be fully charged
at 4.24V. The center of each of the batteries was penetrated from above using a nail
formed of iron and having a diameter of 2.5 mm using a nail penetration tester, and
ignition of the batteries was measured.
[0082] Here, a penetration rate of the nail was constant as 12m/min., and results thereof
are illustrated in Table 2 below.
(Table 2)
|
Ignition number |
Highest temperature of non-ignition sample(°C) |
Inventive Example 1 |
1 |
78 |
Inventive Example 2 |
0 |
42 |
Com parative Example 1 |
5 |
- |
Com parative Example 2 |
5 |
- |
Com parative Example 3 |
5 |
- |
[0083] As illustrated in Table 1, it can be seen that the rechargeable batteries using the
positive electrode according to the present invention are reduced in a short-circuit
area, and thus, a short-circuit current is reduced to enhance safety. In particular,
it can be seen that, when crystallinity was 58.5 or greater by setting the vacuum
dry temperature to 190°C, ignition rarely occurred.
<Comparative Example 4>
[0084] The first positive electrode active material slurry prepared in Comparative Example
1 was applied to have a thickness of 150
µm on an aluminum foil, dried at a rate of 0.2m/min. in a dryer having a temperature
of 130°C under an air atmosphere, and dried again for 24 hours at 160°C in a vacuum
state to form a positive electrode composite layer. The positive electrode composite
layer was then rolled to manufacture a positive electrode.
<Comparative Example 5>
[0085] A positive electrode was manufactured in the same manner as that of Comparative Example
1, except that the first positive electrode active material slurry prepared in Comparative
Example 1 was applied to have a thickness of 50
µm on an aluminum foil and NMP was dried at a rate of 0.2 m/min. in a dryer under an
air atmosphere having a temperature of 130°C and dried again for 24 hours at 160°C
in a vacuum state to form a fist positive electrode composite layer, and a second
positive electrode active material slurry was applied to have a thickness of 100
µm on the fist positive electrode composite layer, dried at a rate of 0.2 m/min. in
a dryer having a temperature of 130°C under an air atmosphere, and dried again for
24 hours at 160°C in a vacuum state to form a positive electrode composite layer.
<Experimental Example 3>
[0086] The positive electrode of Comparative Example 4 was vacuum-dried again at 45°C and
a portion of the electrode layer was scraped out with a razor blade, and NMR of powder
was measured. An analysis method is as follows. After measurement, areas of peaks
of crystalline and non-crystalline at main peaks of the PVDFs were obtained and a
percentage (%) of the area of the crystalline in the sum of the areas was calculated
to obtain crystallinity.
[0087] A used device was Agilent 600MHz NMR/ 1.6mm MAS probe.
[0088] According to the measurement results, crystallinity was 59.7. That is, it can be
seen that, when drying was performed again at 160°C in the vacuum state, crystallinity
of the PVdF was 59.7.
Evaluation of Output Characteristics
[0089] The rechargeable battery manufactured according to Experimental Example 2 using the
positive electrodes manufactured in Inventive Examples 1 and 2 and Comparative Examples
4 and 5 was charged to 4.2V by 0.1C and discharged to 2.5V with 0.1C during two cycles,
and thereafter, it was charged to 4.2V with 0.33C and discharged to SOC 50 with 0.33C
and resistance was measured for 30 seconds at SOC 50 with 3C. Results thereof are
illustrated in Table 3 below.
(Table 3)
|
output(mohm) |
Inventive Example 1 |
1.87 |
Inventive Example 2 |
1.95 |
Comparative Example 4 |
1,92 |
Comparative Example 5 |
1.97 |
[0090] Referring to Table 3, it can be seen that the batteries using the positive electrodes
of Comparative Examples 4 and 5 have high resistance, relative to Inventive Example
1. In addition, it can be seen that Comparative Example 5 has high resistance, as
compared with Inventive Example 2 in which the first positive electrode active material
was vacuum-dried at a higher temperature.
[0091] This is because, in the case of the positive electrodes manufactured according to
Comparative Examples 4 and 5, the second positive electrode composite layers also
underwent a vacuum-drying process at high temperatures, and thus, crystallinity of
the PVdF was increased (higher than 58), and accordingly, overall resistance was increased.
[Industrial Availability]
[0092] As described above, since the electrode for a rechargeable battery of the present
invention uses the binder having crystallinity of 58 or higher in a partial electrode
composite layer forming the multi-layered electrode, elongation percentage of the
multi-layered electrode may be reduced to reduce a short-circuit area at the time
of needle-shaped penetration and increase IR resistance, and thus, safety of the battery
may be effectively enhanced without an additional process or material.